The search for, and evaluation of, unconventional reservoirs of hydrocarbons has become increasingly important in the global energy market. These unconventional reservoirs, such as shale-oil reservoirs, may include crude oil, kerogen, and bitumen. Kerogen, which ultimately generated the crude oil in the reservoir, contains organic compounds that are insoluble in organic solvents. Bitumen is a viscous intermediate hydrocarbon with properties in between those of crude oil and kerogen. Bitumen is often present in source rocks that have begun to generate oil. When these source rocks undergo catagenesis, the kerogen is cracked into smaller molecules to produce bitumen. Similarly, the bitumen may further crack into smaller molecules to produce crude oil.
Petroleum geochemists use the term extractable organic matter (EOM) to describe the petroleum that can be extracted from a thermally-mature oil-prone source rock using an organic solvent. EOM is a mixture of the crude oil generated and retained by the source rock, plus a heavier petroleum phase (“bitumen”) that the kerogen also generated. Bitumen is enriched in non-hydrocarbon compounds that contain nitrogen, sulfur, and/or oxygen (“NSO” compounds) and asphaltenes. It subsequently cracks to lighter, less viscous producible crude oil. Bitumen probably is too viscous to be efficiently expelled from a source rock, although some oil-prone source rocks that contain sulfur-rich kerogen expel heavy tarry oil that may resemble bitumen. Some bitumen and producible oil also may be dissolved in—or sorbed on—kerogen. The kerogen may eventually expel producible oil (and possibly some bitumen) into adjacent intergranular porosity.
Because existing technologies to analyze core samples cannot discriminate between these types of hydrocarbons, there is not an effective way to estimate the quantity and quality of potentially recoverable hydrocarbons in these reservoirs. The analytical techniques petroleum engineers and geochemists currently use to measure the amount of oil in shale-oil reservoirs do not adequately distinguish between producible oil and bitumen. In the same way, the physical and chemical properties and value of the producible oil in a reservoir cannot be determined. Those properties may include API gravity and sulfur content. API gravity, defined by the American Petroleum Institute, is a measure for denoting the density of crude oil. It is an inverse measure; therefore, the lighter the crude, the higher the API gravity, and vice versa. Producible oil with a high API gravity and low sulfur content, which are both desirable characteristics for oil, may have a significantly different economic value from oil with a low API value and high sulfur content.
Core analysis procedures involve using an organic solvent (e.g., toluene) to extract all of the oil and bitumen in a core sample. Shale rock property (SRP) calculations of producible oil saturation (So) probably include the amount of producible oil and bitumen. Geochemists may use programmed pyrolysis to characterize source rocks by using a Source Rock Analyzer (SRA™) or by Rock Eval™. These involve the programmed temperature heating of a small sample of source rock in an inert atmosphere to volatilize or pyrolyze soluble and insoluble organic matter and measure the amount of HC compounds and carbon dioxide produced at different temperatures. These measurements can be used to estimate the amount of oil and residual kerogen in shale-oil samples. Producible oil may consist principally of distillable (S1) compounds, while bitumen and kerogen probably are more enriched in pyrolyzable (S2) compounds. However, producible oil and bitumen probably contribute to both pyrolysis peaks. Likewise, petrophysical log estimates and NMR laboratory measurements of producible oil saturation probably do not adequately resolve producible oil from non-producible bitumen or kerogen.
It is expensive to develop and manage shale-oil reservoirs. Accordingly, proper analysis and evaluation of shale-oil core samples can be of utmost importance in selecting locations and reservoirs to develop. The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.